Conventional designs for high-speed optical transceivers, such as a 400G transceiver, often try to place the physical layer Integrated Circuit (IC) chip as close to the connector as possible in an effort to minimize electrical losses. There is often a Transmitter Optical Sub-Assembly (TOSA) that includes the optical receptacle which must be placed at the front of the transceiver. In many designs, the TOSA has to be rigidly mounted at the front of the transceiver and, to avoid being mechanically over constrained, is connected to the Printed Circuit Board (PCB) by a flex circuit. The flex circuit may be hot bar soldered to the PCB. The PCB pad size required to make a hot bar connection is many times bigger than the signal trace and is a major impedance disruption. There is often excess solder which adds to the impedance discontinuity.
The laser, which may be an Electro-absorption Modulated Laser (EML) in some solutions, is mounted on ground and requires the amplifiers to be connected thru a bias tee. Bias tees are sometimes required because the substrate to the modulator requires a negative bias voltage. The logic conventionally used for drivers, CMOS and SiGe, have difficulty working with negative voltages and the common solution of a bias tee allows an amplifier working at positive voltages, to be AC coupled to the modulator while the modulator is being biased at a negative voltage. The bias tee is physically large compared to the trace, thereby disrupting the impedance and moving the amplifier farther away. The impedance disruptions of the bias tee, hot bar soldered flex as well as other geometry issues almost entirely close a PAM4 eye diagram regardless of the quality of the EML or amplifier. Not only are the bias tees introducing performance problems to the transceiver, but they are a costly component in the overall bill of materials for the transceiver.